Outboard engine units have an exhaust passage system for discharging exhaust gas from an engine, one example of which is disclosed in Japanese Patent Application Laid-Open Publication No. HEI-05-246389(JP H05-246389 A).
FIG. 7 is a view explanatory of the outboard engine unit 100 disclosed in JP H05-246389 A, which has an exhaust passage system 101 for discharging engine exhaust gas into the water (i.e., sea water, river water or the like). More specifically, in the disclosed outboard engine unit 100, exhaust gas from the engine passes through an engine-side exhaust pipe 102 as indicated by arrow (1) and is then directed into an exhaust expansion chamber 103. The exhaust gas is then directed from the exhaust expansion chamber 103 into a downstream exhaust pipe 104 to pass through the downstream exhaust pipe 104 as indicated by arrow (2) and is finally discharged through an exhaust port 106 of a propeller 105.
During operation of the engine, pressure within the exhaust passage system 101 would fluctuate due to variation in the number of rotations of the engine, so that the exhaust gas may flow back in the exhaust passage system 101. If the exhaust gas having water mist mixed therein flows back in the exhaust passage system 101, it may undesirably cause corrosion of metal component parts of the exhaust passage system 101.
Because the exhaust expansion chamber 103 is provided between the engine-side exhaust pipe 102 and the downstream exhaust pipe 104, the outboard engine unit 100 can prevent water from getting into an area which is located upstream of the engine-side exhaust pipe 102 and in which metal component parts are provided.
However, because of the provision of the exhaust expansion chamber 103 and a bent shape of the downstream exhaust pipe 104, the exhaust passage system 101 in the outboard engine unit 100 has a complicated construction and a large size.